Cartridge locks, as mentioned above, are often used to lock meter covers and valve levers. About the end of the cartridge lock opposite the enlarged head are means which couple with complementary elements within a locking cap or bracket to hold the cartridge lock thereto when in its locked position. The enlarged head, together with the device securing the opposite end in some instances, form a barrier past which the element or elements locked cannot pass.
Such locking means include locking balls which, upon locking, are moved to and held in a position partially protruding through apertures in the lock's housing wall. Protruding portions of the locking balls are received by, and contained within, internal pockets in the locking cap or bracket, restraining the movement between these elements that they bridge.
Prior to the present invention, the normal means used to move such locking balls outward to the locked position has been a plunger disposed within an axial core of the cartridge lock, such as the plungers described in U.S. Pat. Nos. 3,186,196 issued June 1, 1965, and 4,015,456 issued Apr. 5, 1977. Along such plungers are two lengths of different circumference and a sloped length therebetween. When the plunger is disposed so that the length of smaller diameter is aligned with the locking balls, there is sufficient space next to the plunger to contain the entire of the locking balls within the lock housing. When the plunger is moved, aligning the larger circumference length with the balls, the balls are pushed partially through the apertures in the housing. Such plungers are typically spring biased in this latter locked position.
To unlock such plunger-type of cartridge locks, unlocking tools are inserted at the free end (enlarged head), which tools are designed to grab the plunger and move it against the spring bias to the unlocked position. The variations in unlocking tools for such type of lock is limited, and thus such systems provide little, if any, master keying potential. In a master keyed system, a number of means for unlocking a single or limited series of locks are distributed to those responsible for that lock or locks while a master means is retained for a number of such locks or series. Loss or misuse of the limited unlocking means confines the security breach, while the holder of the master means retains access to all the locks. Further, given that variations in the unlocking tools for plunger type locks is limited, practicality demands that a given tool operate a significant number of locks, requiring a tight security control of all such tools.
It is an object of the present invention to provide a cartridge lock having the potential for a significant number of combinations of unlocking means. It is an object to provide a cartridge lock with master keying potential. It is an object to provide these features in a cartridge lock of the same or similar outer dimensions as typical plunger type locks in use so that substitution requires no modification of the elements designed to recieve these locks. These and other objects and advantages of the invention are described in more detail below.